There are two ways to persuade your boss to develop a community.
First, use powerful anecdotal evidence. The metrics of a community usually aren’t thrilling. Communities are smaller that followings. It’s hard to link it to direct income generation. Overwhelm your boss with amazing anecdotal stories.
Talk about what the people in your community are likely to do. Bring up specific stories of what’s happened in other communities. Show how you’re falling behind your competitors (or have a rare chance to get ahead).
Second, don’t persuade your boss. Just do it. I’m amazed at just how many communities at very large organizations have been built on the sly. Sometimes it’s just easier to show your boss than to tell your boss. Risky? Sure. Probably worthwhile too.